Generally, shape memory alloys are metal alloys that may be deformed to a "set" shape and otherwise conditioned during processing in such a manner that they may be selectively "actuated" during use to revert or attempt to revert to their pre-deformation shape. In other words, shape memory alloys can "remember" their pre-deformation shape and be selectively activated to move positionally or apply pressure (e.g., to another object), thereby rendering shape memory alloys attractive for actuator and other like applications. To date, proposed shape memory alloys have most typically been thermomechanically conditioned with the alloy starting at least partially in a martensitic state and with thermal cycling driving any phase transformation(s) occurring during conditioning.
Actuation of shape memory alloys is achieved by heating a conditioned alloy to at least a corresponding martensitic-austenitic transformation temperature (e.g., wherein needle-like crystals present in the martensite phase transform to more equi-dimensional crystals characterizing the austenite phase). In this regard, the transformation from a martensitic state to an austenitic state occurs over a range of temperatures, with the "starting" austenitic temperature (A.sub.s) being the temperature at which an austenitic phase for the basic alloy begins to form and coexist with a martensitic phase. The "finish" austenitic temperature (A.sub.f) is the temperature at which the basic alloy is substantially in its austenitic phase. Upon cooling, the basic alloy will change from the austenitic state back to a martensitic state, with the austenitic-martensitic phase transformation also occurring over a range of temperatures. As will be appreciated, the "starting" martensitic temperature (M.sub.s), (i.e., where the martensite phase in the basic alloy begins to form and coexist with the austenite phase), and the "finishing" martensitic temperature (M.sub.f), (i.e., where the basic alloy substantially comprises the martensitic phase) are both lower than the starting austenitic A.sub.s temperature.
The martensitic-austenitic and austenitic-martensitic phase transformation temperature ranges for shape memory alloys vary widely by alloy type/composition and can be varied by alloy conditioning. In this regard, transformation temperatures have been observed as low as about -60.degree. C. and as high as several hundred degrees C. Such a range of transformation temperatures facilitates the potential use of shape memory alloys in a variety of actuator and other like applications.
To date, however, shape memory alloys have not been widely employed in high-precision applications due to reliability and control issues. More particularly, known shape memory alloys display significant hysteresis variability in repeated or cyclic actuations. In this regard, for spacecraft and other applications, it is typically important for actuator devices to respond in "ground-based" testing in a manner which supports a high degree of confidence that the same response will be repeated during actual use. Further, many known shape memory alloys exhibit a relatively wide range in phase transformation temperatures, thereby making it difficult to precisely control actuation.